Morenocetus parvus Cabrera, 1926 (Cetacea: Mysticeti) from the early Miocene of Patagonia, Argentina: a key taxon in the evolutionary history of balaenids.

MÓNICA. R. BUONO; MARTA S. FERNÁNDEZ; MARIO A. COZZUOL

Puerto Madryn

Congreso; 15a Reunión de Trabajo de Expertos en Mamíferos Acuáticos de América del Sur y 9º Congreso de la Sociedad Latino Americana de Especialistas en Mamíferos Acuáticos (SOLAMAC); 2012

The Balaenidae family comprises large and skim feeding baleen whales represented by four living species. Balaenids has been considered a key group in the understanding of Mysticeti evolution due to it is considered the oldest surviving family of Mysticeti. This assumption is based in its early stratigraphic occurrences and old mtDNA estimates divergence times between balaenids and the remaining mysticetes. The earliest balaenid known is Morenocetus parvus from the early Miocene (Gaiman Formation, 20 Ma) of Patagonia. After its original description Morenocetus has not been reviewed, and its phylogenetic relationships remain unresolved. Morenocetus is a critical taxon not only to assist in resolving the balaenid phylogenetic relationships but also for its potential to help resolve higher mysticete relationships. Besides, the study of the earliest balaenids allows us to identified heterochronic patterns in the evolution of balaenids. A heuristic parsimony analysis of 25 species and 123 morphological characters was conducted using traditional search. It resulted in one most parsimonious tree, with a tree length of 310, CI of 0.65, and RI of 0.72, under equal weighting. Morenocetus was recovered as the basalmost taxon of Balaenidae, and as the sister taxon of the clade that includes the remaining balaenids (Balaenula, Balaena, Balaenella and Eubalena). This topology was supported by three unequivocal synapomorphies: squamosal dorsoventrally developed; presence of bulbous postglenoid process and long and shallow stylomastoid fossa. Plesiomorphic characters observed in Morenocetus include e.g. the small size, a supraorbital process of the frontal mostly expanded in lateral plane with moderate downward deflection, an obtuse angle between anterior and posterior process of the periotic, lack a hypertrophied lateral projection of the anterior process and absence of a pyramidal process. Phylogenetic analysis suggests that the position of Morenocetus is in accordance with its early stratigraphic occurrence and implies a long ghost lineage for balaenids. In addition, it reinforces the hypothesis that Balaenidae is an early-divergent lineage of surviving Mysticeti. Finally, the cranial morphology of the calves of Eubalaena australis resembles the condition of their ancestor, Morenocetus, suggesting that extant balaenids evolved by peramorphic mechanisms.